The present invention is generally related to control systems and techniques for controlling compression ignition engines, and, more particularly, the present invention is related to a control system and method for reducing exhaust emissions and improving engine efficiency, specially during partial load conditions of the engine.
In a typical turbocharged or supercharged diesel engine, the engine inlet air may be compressed in a turbocharger or supercharger compressor and then passed through an intercooler to a respective intake manifold prior to being drawn into a respective cylinder of the engine. As will be appreciated by those skilled in the art, a locomotive engine may be operable at a plurality of different throttle positions or notches. In general, a notch specifies a commanded engine speed and/or power and results in a given load and/or speed condition for the engine. For example, there may be eight power positions or notches (N), plus idle. Notch N1 may correspond to the minimum desired engine speed and power, while notch N8 may correspond to maximum speed and full power.
When the engine is operating at or near full speed and load, the temperature of the air discharged by the compressor may be high relative to a coolant fluid circulated through the intercooler, and thus such air would be desirably cooled in the intercooler. Conversely, while the engine operates during partial load conditions, e.g., from notch N1 to a predetermined intermediate notch position or at the idle notch position, the temperature of the compressor-discharge air may be low compared to the intercooler coolant. Under presently known engine control techniques used during partial engine load conditions, the compressed air may be conditioned in the intercooler to about the same temperature level of the engine cylinders coolant and lubrication oil regardless of whether a resulting intake manifold air temperature (IMAT) corresponds to a reduced level of exhaust emissions or an optimal combustion efficiency in the cylinders of the engine, that is, such techniques do not allow for adjusting IMAT to optimize in-cylinder combustion and heat release and consequently exhaust emissions and engine efficiency may be somewhat less than optimal. Thus, such presently known control techniques do not take advantage of the fact that engine output performance and exhaust emissions may be positively affected by appropriate control of IMAT at partial or relatively low load conditions. For example, as the engine is operated in a respective partial load condition, selectably lowering manifold air temperature could reduce engine exhaust emissions including not only nitrogen oxides (NO.sub.x), but also carbon monoxide (CO), smoke and other pollutants.
In view of the foregoing considerations, it is desirable to reduce exhaust emissions and improve the output performance of such engines particularly when operating at partial load conditions.